The present invention relates to an automatic transmission for an automobile and more particularly to an automatic transmission having a plurality of transmission gear trains between input and output shafts.
Generally, a manual transmission, in which the gear is manually shifted, has an input shaft directly connected to an engine and including a plurality of drive gears and has an output shaft including a plurality of driven gears paired with the corresponding drive gears and connected to drive wheels. That is, there are provided a plurality of transmission gear trains between the input shaft and the output shaft. In this manual transmission, when the gear is changed, after a clutch is disengaged, changeover mechanisms such as synchromesh mechanism are manually operated to change over paired transmission gear trains (hereinafter, referred to as just gear trains) and then the clutch is engaged. This sequence of manual operations accomplishes a gearshifting of a vehicle.
The manual transmission can be converted into an automatic transmission by performing gearshiftings and clutch operations by means of a shift actuator hydraulically operated. This type of transmission having a plurality of transmission gear trains is called an Automated Manual Transmission (hereinafter referred to as xe2x80x9cAMTxe2x80x9d). The transmission has advantages of having a small number of components and an excellent transmission efficiency of power, compared to a conventional torque converter type automatic transmission primarily constituted by planetary gears and the like.
For example, Japanese Patent Application Laid-open No. Toku-Kai 2000-65199 discloses this AMT type automatic transmission having a start clutch for changing over the connection of a crank shaft and an input shaft between an engagement condition and a disengagement condition and a changing clutch (hydraulically operated multiple disc clutch) for transmitting torque from the input shaft to an output shaft to prevent a so-called xe2x80x9ctorque dropxe2x80x9d.
According to this disclosure, gear trains of the first gear through the fifth gear are provided between the input and output shafts. Gear trains of third and fourth gear are engaged by a first synchromesh mechanism, respectively and gear trains of first and second gear are engaged by a second synchromesh mechanism, respectively. A gear train of fifth gear is engaged by the engagement of a by-pass clutch. That is, power is transmitted by engaging the by-pass clutch to prevent a torque drop between the input shaft and the output shaft. In this case, since the by-pass clutch is provided in the gear train of fifth gear, the torque capacity transmitted from the input shaft to the output shaft corresponds to that of the fifth gear.
Accordingly, when the gear is changed from the first gear to the second gear, the torque transmitted through the bypass clutch is a transmission torque at the fifth gear ratio. As a result, the transmission torque drops from the torque of the second gear to that of the fifth, even if an inertia force of the bypass clutch increases the transmission torque in some degree. This torque drop, or shift shock has a great effect on xe2x80x9cshift qualityxe2x80x9d of the transmission. On the other hand, in order to reduce the magnitude of shift shock, when the shift time is elongated, a driver feels uncomfortable. Similarly, when the gear is changed from the second gear to the third gear, the driver still feels a shift shock.
Some vehicle has an automatic transmission being equipped with an extra high gear position, for example, a sixth gear position or an extra low gear position for the purpose of improving fuel economy, enhancing quietness in a high speed traveling or increasing driving force of a four wheel drive vehicle. In this case, the shift quality of the transmission may further be exacerbated by an increased difference of gear ratio between the lowest and highest gear positions. In particular, in case where such transmission is installed on an industrial vehicle or a heavy duty vehicle, the discrepancy of gear ratios increases and shift shock further becomes worse.
Japanese Patent Application Laid-open No. Toku-Kai-Hei 4-262166 discloses an automatic transmission in which two load connecting clutches are provided between the input and output shafts. One load connecting clutch transmits power to the gear train at even number gear positions, second and fourth gear and the other load connecting clutch transmits power to the gear train at odd number gear positions, first, third and fifth gear ratios. When the gear is shifted, a changeover is performed between two load connecting clutches. For example, when the gear is up-shifted from the first gear to the second gear, since the load connecting clutch on the even number gear position side is disengaged, a synchronizer sleeve is operated beforehand to engage the synchronizer for the second gear ratio with the output shaft. Next the load connecting clutch on the odd number gear position side is disengaged and the load connecting clutch on the even number gear position side is engaged. Thus, the gear shifting form the first to second gears is accomplished.
However, since this automatic transmission is designed so as to shift gears by changing over the load connecting clutch between the odd number gear position side and the even number gear position side, in case where the gear is up-shifted from the first gear ratio to the fifth gear ratio, it is necessary to step in midway gear positions, second, third and fourth positions in sequence. Therefore, for example, such jumping shifts as shifts from first gear to third gear or second gear to fourth gear, are impossible. Since the gear must be changed while midway gear positions are experienced, it takes a long time to shift gears. Especially, in case of passing, when a gearshifting must be finished in short time, the gearshifting taking time gives a psychological burden on a driver. Such inconvenience is remarkable particularly when the vehicle travels on winding roads with up-and-downs.
It is an object of the present invention to provide an automatic transmission having small shift shocks and a good shift quality. It is a further object of the present invention to realize a compact automatic transmission having an excellent installability. Further, it is another object of the present invention to provide an automatic transmission capable of making a jumping shift arbitrarily. Furthermore, it is still another object of the present invention to provide an automatic transmission having a good shift responsibility.
In order to achieve these objects, an automatic transmission having a plurality of gear trains provided between an input shaft and an output shaft for transmitting torque from the input shaft to the output shaft, the gear trains including drive gears mounted on the input shaft and driven gears mounted on the output shaft, comprises a start clutch provided between a crankshaft of an engine and the input shaft for transmitting or shutting off engine power from a crankshaft of the engine to the input shaft, a changeover mechanism for selecting a desired gear train through which power is transmitted from a plurality of gear trains, a plurality of bypass clutches provided between specified gear trains for transmitting a different torque from the input shaft to the output shaft respectively, and a bypass clutch control means for selecting at least one desired bypass clutch through which power is transmitted when the gear is shifted from a plurality of the bypass clutches.